FIG. 1 is a cross-sectional view of a wind turbine rotor blade 10. The blade has an outer shell, which is fabricated from two half shells: a windward shell 11 a and a leeward shell 11b. The shells 11a and 11b are typically moulded from glass-fibre reinforced plastic (GRP). Parts of the outer shell 11 are of sandwich panel construction and comprise a core 12 of lightweight material such as foam (e.g. polyurethane) or balsa, which is sandwiched between inner 13 and outer 14 GRP layers or ‘skins’. Other core materials will be apparent to persons skilled in the art.
The blade 10 comprises a first pair of load-bearing structures in the form of spar caps 15a and 15b and a second pair of load-bearing structures in the form of spar caps 16a, 16b. The respective pairs of spar caps 15a and 15b, 16a and 16b are arranged between sandwich panel regions of the shells 11a and 11b. One spar cap 15a, 16a of each pair is integrated with the windward shell 11a and the other spar cap 15b, 16b of each pair is integrated with the leeward shell 11b. The spar caps of the respective pairs are mutually opposed and extend longitudinally along the length of the blade 10.
A first longitudinally-extending shear web 17a bridges the first pair of spar caps 15a and 15b and a second longitudinally-extending shear web 17b bridges the second pair of spar caps 16a and 16b. The shear webs 17a and 17b in combination with the spar caps 15a and 15b and 16a and 16b form a pair of I-beam structures, which transfer loads effectively from the rotating blade 10 to the hub of the wind turbine. The spar caps 15a and 15b and 16a and 16b in particular transfer tensile and compressive bending loads, whilst the shear webs 17a and 17b transfer shear stresses in the blade 10.
Each spar cap 15a and 15b and 16a and 16b has a substantially rectangular cross section and is made up of a stack of pre-fabricated reinforcing strips 18. The strips 18 are pre-cured pultruded strips of carbon-fibre reinforced plastic (CFRP), and are substantially flat and of rectangular cross section. The number of strips 18 in the stack depends upon the thickness of the strips 18 and the required thickness of the shells 11a and 11b, but typically the strips 18 each have a thickness of a few millimetres and there may be between three and twelve strips in a stack. The strips 18 have a high tensile strength, and hence have a high load bearing capacity.
The blade 10 is made using a resin-infusion process as will now be described by way of example with reference to FIGS. 2 and 3. Referring to FIG. 2, this shows a mould 20 for a half shell of a wind turbine blade in cross-section. A glass-fibre layer 22 is arranged in the mould 20 to form the outer skin 14 of the blade 10. Three elongate panels 24 of polyurethane foam are arranged on top of the glass-fibre layer 22 to form the sandwich panel cores 12 referred to above. The foam panels 24 are spaced apart relative to one another to define a pair of channels 26 in between. A plurality of pultruded strips 18 of CFRP, as described above with reference to FIG. 1, are stacked in the respective channels 26. Three strips 18 are shown in each stack in this example, but there may be any number of strips 18 in a stack.
Referring to FIG. 3, once the strips 18 have been stacked, a second glass-fibre layer 28 is arranged on top of the foam panels 24 and the stacks of pultruded strips 18. The second glass-fibre layer 28 forms the inner skin 13 of the blade 10. Next, vacuum bagging film 30 is placed over the mould 20 to cover the layup. Sealing tape 32 is used to seal the vacuum bagging film 30 to a flange 34 of the mould 20. A vacuum pump 36 is used to withdraw air from the sealed region between the mould 20 and the vacuum bagging film 30, and resin 38 is supplied to the sealed region. The resin 38 infuses between the various laminate layers and fills any gaps in the laminate layup. Once sufficient resin 38 has been supplied to the mould 20, the mould 20 is heated whilst the vacuum is maintained to cure the resin 38 and bond the various layers together to form the half shell of the blade. The other half shell is made according to an identical process. Adhesive is then applied along the leading and trailing edges of the shells and the shells are bonded together to form the complete blade.
The integration of the spar caps 15a and 15b and 16a and 16b within the structure of the outer shells 11a and 11b avoids the need for a separate spar cap such as a reinforcing beam, which is typically bonded to an inner surface of the shell in many conventional wind turbine blades. Other examples of rotor blades having spar caps integral with the shell are described in EP 1 520 983, WO 2006/082479 and UK Patent Application GB 2497578.
When manufacturing wind turbine blades using a resin infusion process, it is important to control the resin flow front during the infusion process to ensure that the resin infuses evenly and completely throughout the laminate layup and between all of the shell components. If the flow front is not carefully controlled, then air pockets (also referred to as ‘lock offs’ or voids) may develop in the blade structure. Air pockets are caused by the incomplete infusion of resin in certain regions of the blade, and can result in localised weaknesses in the blade structure.
The present invention has been developed against this background, and provides an improved method of manufacturing a wind turbine blade. In particular, the invention provides increased control over the resin flow front during resin infusion and eliminates or at least significantly reduces the possibility of air pockets forming. The present invention resides both in the identification of the problem, and in the solution to the problem.
The particular problem identified by the inventors will now be described in detail with reference to FIGS. 4 to 8.
FIG. 4 is a schematic representation of a spar structure 40 for a wind turbine blade arranged between first and second foam panels 42a and 42b. Referring to FIG. 4, the spar structure 40 in this example is a spar cap and comprises a plurality of CFRP pultrusions 44 arranged one on top of another to form a stack. The foam panels 42a and 42b are made from polyurethane foam. The spar cap 40 and foam panels 42a and 42b are arranged side by side in a suitable mould, for example a wind turbine blade shell mould (not shown), as described previously by way of introduction with reference to FIG. 2. Both the spar structure 40 and the foam panels 42a and 42b extend longitudinally in the mould, in a generally spanwise direction. A resin inlet channel 46 is also shown in FIG. 4, and will be described in further detail later with reference to FIG. 7.
As shown in FIG. 4, a small gap 48 is present on each side of the spar cap 40, between the spar cap 40 and the adjacent foam panel 42a or 42b. Whilst the spar caps 40 and foam panels 42a and 42b are arranged in the mould in close abutment, a small gap 48 is inevitable for reasons as will now be explained with reference to FIGS. 5 and 6.
FIG. 5 is a schematic representation of a transverse cross section taken through a wind turbine blade shell mould 50. A spar cap 40 and adjacent foam panel 42 are also shown schematically inside the mould 50. The blade shell mould 50 has a concave curvature generally in the chordwise direction C, corresponding to part of the airfoil profile of the blade to be produced. The curvature of the mould 50 prevents the spar cap 40 and foam panel 42 from abutting closely across the entire interface 52 between the two components 40 and 42, and results in a longitudinally-extending gap 48 at the interface 52.
Referring now also to FIG. 6, this is a schematic representation of part of the spar cap 40. Here it can be seen that there may be a slight misalignment between the stacked pultrusions 44 comprising the spar cap 40. The misalignments are exaggerated for clarity in FIG. 6, and in practice any misalignment may only be a fraction of a millimetre. In any event, misalignment between the stacked pultrusions 44 results in the longitudinal sides of the spar cap 40 not being perfectly flat, and this also contributes to the longitudinally-extending gaps 48 between the spar cap 40 and the adjacent foam panel 42 at the interface 52 between the abutting components 40 and 42.
The gaps 48 described above may cause undesirable resin flow during the infusion process as will now be described with reference to FIGS. 7 and 8.
Referring to FIG. 7, during the resin infusion process, resin is admitted into the mould via the resin inlet channel 46. The resin inlet channel 46 has a generally omega-shaped cross section, and extends longitudinally and substantially centrally in the mould. Resin is admitted into one end of the channel 46, for example the end 54 shown in cross-section in FIG. 7, and the resin flows along the channel 46 in a generally spanwise direction S. Resin also flows out of the channel 46 in a generally chordwise direction C across the foam panel 42 and spar cap 40 in the mould as represented by the arrows 56 in FIG. 7. The aim of this arrangement is to achieve an angled flow front of the resin across and along the components 40, 42 as represented schematically by the shaded region 58 in FIG. 7.
However, and referring now to FIG. 8, when the resin reaches the longitudinally-extending gaps 48 between the spar cap 40 and the foam panels 42, the gaps 48 act as ‘race tracks’ for the resin, and the resin flows quickly along the gaps 48 in the spanwise direction S. The fast and uncontrolled resin flow along the gaps 48 can result in resin lock offs 60 forming, as shown in FIG. 8. The air contained in the lock off 60 cannot escape and so this region will not be infused. This lock off 60 may be present between individual pultrusions 44 of the spar cap 40.
The present invention provides a solution to this problem in the form of a method of making an elongate wind turbine blade extending longitudinally between a root end and a tip end in a spanwise direction, the method comprising:                a. providing an elongate mould tool extending longitudinally in a spanwise direction;        b. arranging an elongate spar structure in the mould tool, the spar structure extending longitudinally in the spanwise direction;        c. arranging core material adjacent to the spar structure;        d. providing resin-permeable material between the spar structure and the core material; and        e. administering resin into the mould during a resin infusion process,wherein the resin-permeable material restricts the flow of resin between the spar structure and the core material in the spanwise direction.        
Steps b, c and d of the method may be performed in any order.
According to the present invention, resin-permeable material is provided between the spar structure and the core material. The resin-permeable material restricts the flow of resin in the spanwise direction at the interface between the spar structure and the core material as compared to the situation where resin-permeable material is not provided at these interfaces. Thus, the race track effect described above, and the associated resin lock offs, are effectively prevented, and a more controlled resin flow front is achieved in the chordwise direction.
The spar structure referred to above is a load-bearing structure and in preferred embodiments of the invention it is a spar cap comprising a stack of pultruded strips of reinforcing material as described previously. However, it should be appreciated that the invention is not limited in this respect and the spar structure may be another suitable load-bearing structure. The spar structure may be made of pre-cured material. For example the spar structure may be made of carbon-fibre reinforced plastic (CFRP).
The core material may be any suitable core material, for example of the type typically used as the core of sandwich panels. Preferably the core material is foam, for example polyurethane foam, but it may instead be balsa or another suitably-lightweight material. In preferred examples of the invention, the core material is in the form of panels that are arranged in abutment with the spar structure, as described earlier.
The resin-permeable material may be any compliant material that is capable of reducing the flow rate of resin at the interface between the spar structure and the core material. In preferred embodiments of the invention, the material is breather fabric, for example breather fabric made from polyester, nylon or blended fibreglass. Suitable breather fabrics include those produced by Tygavac Advanced Materials Ltd., such as the ‘Econoweave’, ‘Airweave’ and ‘Ultraweave’ series of fabrics. The breather fabric typically has a weight in the range of approximately 100-700 g/m2, although other weights may be suitable. As an alternative to breather fabric, the resin-permeable material may include polystyrene beads, spun polyester, or sponge material. The material will typically undergo some compression during the moulding process, and suitable materials are those that still allow resin to flow (albeit at a reduced flow rate) at the interface between the spar structure and the core material when the resin-permeable material is compressed to such an extent.
The method may involve securing the resin-permeable material to the core material and/or to the spar structure. This has the advantageous effect of maintaining the breather fabric in the desired position during the layup process and during the subsequent infusion process. The resin-permeable material may be secured to the spar structure and/or to the core material when the associated component is arranged in the mould. For example the method may involve arranging the core material in the mould and subsequently attaching the resin-permeable to the core material, for example before the spar structures are arranged in the mould.
A particularly advantageous effect may be realised by pre-attaching the resin-permeable material to the spar structure or to the core material before arranging the blade components in the mould. For example in a particular example of the invention, the resin-permeable material is pre-applied to the core material before the core material is arranged in the mould. This operation can be performed offline and hence reduces the blade production time in the mould. The resin-permeable material may be secured to the core material and/or to the spar structure by any suitable means, for example it may be bonded by a suitable adhesive or secured using scrim tape.
During the resin-infusion process, the method may comprise administering resin into the mould in a direction transverse to the spanwise direction. Preferably the method comprises administering resin into the mould substantially in a chordwise direction, i.e. across the width of the mould.
The method may further comprise providing a resin inlet channel extending longitudinally in the spanwise direction through which the resin is administered into the mould during the resin infusion process, and preferably the elongate spar structure is positioned between the resin-permeable material and the resin inlet channel. This prevents resin lock offs between the spar structure and the core material.
The mould is preferably a blade shell mould. The mould may be a mould for making a half shell of a wind turbine blade. Alternatively the mould may be configured to make an entire wind turbine blade. As a further alternative, the mould may be for making a section of a wind turbine blade, for example in the case of a modular blade. Hence, the method may involve making only part of a wind turbine blade according to the present invention. For example, a mid-section of a blade may be made according to the above method, and the mid-section may subsequently be joined to a root and/or tip portion of the blade, or to another longitudinal section of the blade.
Accordingly, the present invention provides a wind turbine blade made in accordance with the above method, and a wind turbine comprising the wind turbine blade.
The invention therefore provides a wind turbine blade extending longitudinally between a root end and a tip end in a spanwise direction, the wind turbine blade having a blade shell made of fibre-reinforced plastic, and at least part of the blade shell comprising: an integral elongate spar structure extending longitudinally in the spanwise direction; core material arranged adjacent to the spar structure; and resin-permeable material provided between the spar structure and the core material.
The wind turbine blade is formed by resin infusion according to the method described above. During the resin-infusion process, the resin-permeable material serves to restrict the rate of flow of resin between the spar structure and the core material in the spanwise direction. The resin-permeable material substantially fills any gaps at the interfaces between the spar structure and the core material and eliminates the race-track effect at such interfaces.
Optional features described above in relation to the method are equally applicable to the invention when expressed in terms of a wind turbine blade, but these features will not be repeated herein for reasons of conciseness.